Does the Principled Shader IOR value effect specular reflection on a transmission material
TL;DR:
the specular value controls the IOR for reflection, the IOR value is indeed just used for refraction.

Background info:
The principled BSDF-Shader is based off of disneys principled BRDF.
It's called principled because of these 5 principles:


Intuitive rather than physical parameters should be used.
There should be as few parameters as possible.
Parameters should be zero to one over their plausible range.
Parameters should be allowed to be pushed beyond their plausible range where it makes sense.
All combinations of parameters should be as robust and plausible as
possible.


Because of their first principle (easy instead of scientific) Disney decided to cloak the fresnel-IOR in a specular reflection value. This is of course a more artistic approach.

from the disney docs: specular - incident specular amount. This is in lieu of an explicit index-of-refraction 

and from the blender docs: Specular
Amount of dielectric specular reflection. Specifies facing (along normal) reflectivity in the most common 0 - 8% range.

You can use the formula =((−1)/(+1))2/0.08 to create a node group so you can control both IORs with one slider.

Note that the specular value accepts values above 1.